# std::enable_shared_from_this

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< cpp‎ | memory

Defined in header `<memory>` |  |
template< class T > class enable_shared_from_this; |  | (since C++11)

`std::enable_shared_from_this` allows an object t that is currently managed by a std::shared_ptr named pt to safely generate additional std::shared_ptr instances pt1, pt2 etc. that all share ownership of t with pt.

Publicly inheriting from `std::enable_shared_from_this<T>` provides the type `T` with a member function `shared_from_this`. If an object t of type `T` is managed by a std::shared_ptr<T> named pt, then calling `T::shared_from_this` will return a new std::shared_ptr<T> that shares ownership of t with pt.

### Data members

Member | Description
mutable std::weak_ptr<T> `weak_this` | the object tracking the control block of the first shared owner of *this (exposition-only member object*)

### Member functions

(constructor) | constructs an `enable_shared_from_this` object (protected member function)
(destructor) | destroys an `enable_shared_from_this` object (protected member function)
operator= | returns a reference to *this (protected member function)
shared_from_this | returns a std::shared_ptr which shares ownership of *this (public member function)
weak_from_this (C++17) | returns a std::weak_ptr which shares ownership of *this (public member function)

### Notes

The constructors of std::shared_ptr detect the presence of an unambiguous and accessible (i.e. public inheritance is mandatory) `enable_shared_from_this` base and assign the newly created std::shared_ptr to `weak_this` if not already owned by a live std::shared_ptr. Constructing a std::shared_ptr for an object that is already managed by another std::shared_ptr will not consult `weak_this` and thus will lead to undefined behavior.

It is permitted to call `shared_from_this` only on a previously shared object, i.e. on an object managed by std::shared_ptr<T>. Otherwise, std::bad_weak_ptr is thrown (by the std::shared_ptr constructor from a default-constructed `weak_this`﻿).

`enable_shared_from_this` provides the safe alternative to an expression like std::shared_ptr<T>(this), which is likely to result in this being destructed more than once by multiple owners that are unaware of each other (see example below).

### Example

Run this code

    #include <iostream>
    #include <memory>

    class Good : public std::enable_shared_from_this<Good>
    {
    public:
        std::shared_ptr<Good> getptr()
        {
            return shared_from_this();
        }
    };

    class Best : public std::enable_shared_from_this<Best>
    {
        struct Private{ explicit Private() = default; };

    public:
        // Constructor is only usable by this class
        Best(Private) {}

        // Everyone else has to use this factory function
        // Hence all Best objects will be contained in shared_ptr
        static std::shared_ptr<Best> create()
        {
            return std::make_shared<Best>(Private());
        }

        std::shared_ptr<Best> getptr()
        {
            return shared_from_this();
        }
    };

    struct Bad
    {
        std::shared_ptr<Bad> getptr()
        {
            return std::shared_ptr<Bad>(this);
        }
        ~Bad() { std::cout << "Bad::~Bad() called\n"; }
    };

    void testGood()
    {
        // Good: the two shared_ptr's share the same object
        std::shared_ptr<Good> good0 = std::make_shared<Good>();
        std::shared_ptr<Good> good1 = good0->getptr();
        std::cout << "good1.use_count() = " << good1.use_count() << '\n';
    }

    void misuseGood()
    {
        // Bad: shared_from_this is called without having std::shared_ptr owning the caller
        try
        {
            Good not_so_good;
            std::shared_ptr<Good> gp1 = not_so_good.getptr();
        }
        catch (std::bad_weak_ptr& e)
        {
            // undefined behavior (until C++17) and std::bad_weak_ptr thrown (since C++17)
            std::cout << e.what() << '\n';
        }
    }

    void testBest()
    {
        // Best: Same but cannot stack-allocate it:
        std::shared_ptr<Best> best0 = Best::create();
        std::shared_ptr<Best> best1 = best0->getptr();
        std::cout << "best1.use_count() = " << best1.use_count() << '\n';

        // Best stackBest; // <- Will not compile because Best::Best() is private.
    }

    void testBad()
    {
        // Bad, each shared_ptr thinks it is the only owner of the object
        std::shared_ptr<Bad> bad0 = std::make_shared<Bad>();
        std::shared_ptr<Bad> bad1 = bad0->getptr();
        std::cout << "bad1.use_count() = " << bad1.use_count() << '\n';
    } // UB: double-delete of Bad

    int main()
    {
        testGood();
        misuseGood();

        testBest();

        testBad();
    }

Possible output:

    good1.use_count() = 2
    bad_weak_ptr
    best1.use_count() = 2
    bad1.use_count() = 1
    Bad::~Bad() called
    Bad::~Bad() called
    *** glibc detected *** ./test: double free or corruption

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2179 (P0033R1) | C++11 | given type `T` derived from `enable_shared_from_this`, the behavior of constructing two std::shared_ptr<T>s from the same T* object was unclear | the behavior is undefined in this case
LWG 2529 (P0033R1) | C++11 | it was unclear how the underlying std::weak_ptr is updated | made clear

### See also

shared_ptr (C++11) | smart pointer with shared object ownership semantics (class template)
make_sharedmake_shared_for_overwrite (C++20) | creates a shared pointer that manages a new object (function template)

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